batman-adv: document cleanup of batadv_wifi_net_devices entries

It doesn't seem to be obvious how the entries from the
batadv_wifi_net_devices rhashtable are getting removed before the actual
rhashtable is destroyed. Document the idea behind the process and which
steps are involved.

Signed-off-by: Sven Eckelmann <sven@narfation.org>

net/batman-adv/hard-interface.c

@@ -1185,5 +1185,9 @@ int __init batadv_wifi_net_devices_init(void)
  */
 void batadv_wifi_net_devices_deinit(void)
 {
+	/* just destroy table. entries should have been removed by
+	 * unregister_netdevice_notifier() and the corresponding
+	 * NETDEV_UNREGISTER events
+	 */
 	rhashtable_destroy(&batadv_wifi_net_devices);
 }